The present invention relates to an offset reducing circuit for a differential amplifier used in driving circuits, such as a CD driver.
Conventionally, a simple operational amplifier as shown in FIG. 9 is employed as a driving circuit such as a CD driver. In the operational amplifier, a differential amplifier 2 is arranged at front end portion. In this case, in the differential amplifier 2, a differential couple 24 is provided in which a resistor 23 connects between emitters of transistors 21 and 22. Constant current sources 25 and 26 are connected to the emitters of the transistors 21 and 22. Namely, in this differential amplifier 2, operational currents are supplied to the transistors 21 and 22 by drawing a constant current I into the constant current sources 25 and 26. At collector side of the transistor 21 and 22, a current mirror circuit 27 is arranged as an active load of the differential couple 24. In other words, a transistor 28 of which base and collector are commonly connected (namely diode connection) is connected between the collector of the transistor 21 and a power source Vcc, while a transistor 29 is connected between the collector of the transistor 22 and the power source Vcc.
In this differential amplifier 2, a transistor 6 is provided to constitute a feeds back loop 4 which feedback an output of the differential couple 24 from the collector of the transistor 29 into the base of the transistor 22. The transistor 6 is connected between the power source and the base of the transistor 22. A base of the transistor 6 is connected to the collector of the transistor 22. A constant current source 8 is connected between the base of the transistor 22 and a reference potential point (grounding point) so that a constant current flowing from the transistor 6 is drawn into the constant current source 8.
In such operational amplifier, when an input signal is applied via a input terminal 10, the input signal is amplified by the differential amplifier 2. The amplified output is applied to the base of the transistor 6 through the transistor 29 of the current mirror circuit 27 to output from an output terminal 12 according to the relationship with the constant current being drawn into the constant current source 8. At the same time, a part of the output current is fed back to the base of the transistor 22. The above mentioned differential amplifier 2 using the differential couple in which emitters are connected by the resistor 23 is frequently used in the case of where it is not necessary to match an amplifier gain by increasing a slew rate by raising the constant current.
In the above-mentioned differential amplifier 2, the constant current may vary due to differences in the transistors which constitute the constant current sources 25 and 26 by reason of the IC manufacturing process. Why the offset can occur will be explained. To simplify the explanation, as shown in FIG. 10A, the feedback loop 4 is simplized, and the transistors 21, 22, 28 and 29 are assumed to be ideal transistors. In the drawing, the transistor 28 is indicated as a diode, but it is a transistor of which the base and the emitter are commonly connected to constitute a current mirror circuit 27 as same as the differential circuit 27 shown in FIG. 9. In this differential amplifier 2, it is assumed that the collector current of the transistor 21 is reversed at the current mirror circuit to cause the current to flow to the collector side of the transistor 22, since the currents of transistors 21, 22, 28, and 29 must be equal so as to balance the currents of the current mirror circuit 27. If the collector currents of the transistors 21 and 22 are represented by I.sub.c1 and I.sub.c2, an equation I.sub.c1 =I.sub.c2 is established. In the case of where the constant current I of the constant current source 25 is increased by .DELTA.I, an equation I.sub.e1 =I.sub.e2 is established from the equation I.sub.c1 =I.sub.c2. Where I.sub.e1 and I.sub.e2 are emitter currents of the transistors 21 and 22. Therefore, the emitter currents I.sub.e1 and I.sub.e2 are expressed by I.sub.e1 =I.sub.e2 =I+.DELTA.I/2. Accordingly, a current flowing through the resistor 23 becomes .DELTA.I/2. If a resistance of the resistor 23 is indicated by R, a voltage (R.times..DELTA.I/2) is generated across the resistor 23, that is, between the emitters of the transistors 21 and 22 due to the voltage drop expressed by a product of the current flowing the resistor 23 by the resistance of the resistor 23. According to the equations I.sub.c1 =I.sub.c2 and I.sub.e1 =I.sub.e2, an offset voltage .DELTA.V (=R.times..DELTA.I/2) is produced between a base of the transistor 22 and a base of transistor 21 which is the same as the voltage (=R.times..DELTA.I/2) between the emitters of the transistors 21 and 22.
In the case where the feedback loop is disconnected and the bases of the transistors 21 and 22 are grounded in the differential amplifier 2 as shown in FIG. 10B, currents flowing through the transistors 21 and 22 are current values detected at the emitter side. A current flowing through the resistor 23 is exponentially compressed according to the voltage difference of the diode 28. Therefore, if the current difference .DELTA.I of the constant current I is small, the current flowing through the resistor 23 can be negligible.
However, as shown in FIG. 10A, since a current due to the current difference .DELTA.I is fed back to the base side of the transistor 22 via the feedback loop 4, I.sub.c1 =I.sub.c2 is established by the feedback operation of .DELTA.I=0. Then, the constant currents I+.DELTA.I and I are equally distributed as currents I+.DELTA.I/2 to the transistors 21 and 22. As a result, an offset voltage is generated.